A pro-Russian militant defends a front line position with a machine gun, Slovyansk, Ukraine, May 19, 2014.

Residents watch the flames from a damaged gas pipe that was hit by a mortar bomb, during fighting between Ukrainian government troops and pro-Russian militants, outside Slovyansk, Ukraine, May 19, 2014.

Pro-Russian militants detain three men they suspect of spying for the Ukrainian government in Kramatorsk, eastern Ukraine, May 18, 2014.

A pro-Ukrainian activist prepares to hoist the Ukrainian flag in the town of Velika Novosyolka, in the Donetsk region, eastern Ukraine, May 17, 2014.

A masked pro-Russian militant stands behind the barricades at a checkpoint blocking the major highway outside Kharkiv, eastern Ukraine, May 17, 2014.